The type I human T cell leukemia virus (HTLV-I) is a complex type C retrovirus etiologically linked with several diverse human diseases including: (1) the Adult T Cell Leukemia (ATL), an aggressive and usually fatal neoplastic expansion of activated CD4+ T lymphocytes; (2) Tropical Spastic Paraparesis/HTLV-I associated myelopathy (TSP/HAM), a progressive neurodegenerative process involving the spinal cord; and (3) a variety of poorly understood autoimmune or inflammatory diseases that affect different end organs including the eye, salivary glands, joints, and muscle. The molecular basis for HTLV-I transformation of human CD4+ T lymphocytes remains unclear, however, the pX-encoded Tax gene product appears to play a central role. Tax both enhances viral replication and deregulates the expression of various host cell genes by altering the activity of select host transcription factors including NF-kappaB/Rel and CREB/ATF. To define the molecular events associated with full T cell transformation, we will utilize Tax expressing transgenic mice that develop T-cell lymphomas closely resembling ATL. Transgenic animals expressing mutants of Tax that are selectively impaired for NF-kappaB/Rel or CREB/ATF dependent gene expression will be evaluated to define the role played by each of these individual transcription factor pathways in T cell transformation. We will further assess the requirement for continuous Tax or NF-kappaB/Rel expression for maintenance of Tax induced tumors by preparing and studying transgenic animals conditionally expressing Tax or a nondegradable mutant of IkappaBalpha that potently inhibits NF-kappaB/Rel activation by Tax (Specific Aim 1). In a second line of study, we will explore how Tax activates the NF-kappaB/Rel family of transcription factors, following its physical assembly with the IKKgamma subunit of the signalsome complex. The potential recruitment of upstream kinases to the signalsome by Tax and the potential role of Tax oligomerization will be assessed. In related studies, we will test whether the inactivation of the p53 tumor suppressor by Tax involves phosphorylation by the IKK complex. (Specific Aim 2). In a third line of experimentation, we will investigate how Tax induces the sustained, long-term nuclear expression of NF-kappaB, which differs sharply from transient response elicited by physiological stimuli. We will specifically examine whether Tax subverts the generation of negative signals within the IKK complex that normally terminates signalsome action. Additionally, we will explore the potential interplay of Tax with a newly recognized negative pathway of nuclear NF-kappaB regulation involving deacetylation of RelA by histone deacetylase 3. Such deacetylation promotes the rapid nuclear export of RelA and contributes to termination of the NF-kappaB response (Specific Aim 3). Through these combined in vivo and in vitro experiments, we hope to gain key insights into the molecular events underlying HTLV-I Tax mediated leukemogenesis.